Cathode ray tube



July 4, w39. R TRUELL CATHODE RAY TUBE Filed June ,19, 1957 -INVENTOR ROHN TRUE/.L

ATTORNEY Patented July 4, 1939 UNETED STATES PATENT OFFiQE CATI-IODE RAY TUBE Delaware Application June 19, 1937, Serial No. 149,044

Claims.

My invention relates to improvements in cathode ray viewing tubes such as oscilloscope and television image recreation devices.

A conventional type of cathode ray oscilloscope 5 comprises an electron gun for forming a sharply focused cathode ray beam, a target assembly including a substantially flat plate coated with fluorescent material to form a screen, and means for deflecting the electron beam to scan the fluorescent screen and thus trace on it a predetermined pattern. In the use of such tubes and especially of those tubes used for the projection of the fluorescent image on a distant viewing screen, it is highly desirable to obtain as much useful light from the fluorescent screen as possible.

In one type of fluorescent screen, often termed a transmission screen, a. layer of fluorescent material is deposited directly upon a glass wall and is viewed through the glass. The light emitted at the surface of the fluorescent material which is bombarded by the electron beam must pass through the layer of iluorescent material and the glass. Such screens have disadvantages, the

principal one being the limitation of the useful light output due to light absorption by the nuorescent material.

In another type of uorescent screen, often referred to as a reflection screen, a layer of'fluoressent material is deposited on a polished metal carrier. For a given beam intensity the useful light output of a reilection screen is greater than that of a transmission screen. The use of reilection screens presents the difficulty that the electron gun, the neck of the tube enclosing the gun, and the beam deflection means are in the path of the light from the screen unless the gun aros makes a considerable angle, usually 45, with the normal to the screen so that the tube neck and the deilection means no longer interfere.

When this angle is sufciently great, there results a distortion of line and of pattern when the screen is scanned by the cathode ray beam. For example, if the beam spot is originally round it becomes elliptical, and if the area swept out by the beam is originally rectangular it becomes trapezoidal. 'I'his type of distortion is usually referred to as keystone distortion. The keystoning of the pattern, inseparable from the use of an inclined reflection screen, may be partially compensated by giving the electron beam greater deflection while focused on the edge of the screen closer to the electron gun than while focused on the opposite edge.

The principal object of my invention is to pro- (Cl. Z50-157) vide a cathode ray tube in which an undistorted pattern is produced on a target electrode and in which compensation for keystoning is not necessary.

A further object of my invention is to provide 5 a cathode ray tube in which an undistorted pattern is produced on a luminescent screen of the reflection type.

A better understanding of my invention will be obtained, and other objects, features, and ad- 10 vantages will appear, from the following description taken in connection with the accompanying drawing in which:

' Figure 1 is a perspective view, partly broken away, of one form of my invention, and 15 Figure 2 is a diagrammatic representation of a tube incorporating a modification of my invention.

The tube shown in Figure 1 comprises a highly evacuated envelope or bulb I with a tubular arm 20 or neck section enclosing a conventional type electron gun and an enlarged portion, preferably a substantially flat cylindrical portion, enclosing a target electrode 2, displaced laterally from the neck of the tube with its plane perpendicular to the electron gun axis and facing a transparent portion or window 3. The electron gun assembly is of the conventional type and comprises a cathode 5, control electrode t connected to the usual biasing battery and a first anode 30 1, maintained positive with respect to the cathode 5 by a battery 8. The electron stream leaving the first anode 1 is accelerated and concentrated into an electron scanning beam 9 by a second anode I0, which is preferably a conductive coat- 35 ing on the inner surface of the envelope l and extending within the cylindrical section but removed from that portion 3 of the envelope through which light from the target 2 may be projected. Conventional electron beam deection 40 means, such as coils Il and l2, are located near the end of the neck of the tubev to generate deection fields which sweep the beam horizontally and vertically. It is obvious that electrostatic deection plates may be substituted for either one 45 or bot-h sets of the deflection coils, if desired. The target 2 is connected directly to the conductive coating forming the second anode IE) and to the-positive terminal of the battery 3.

In accordance with my invention I bend the 50 electron beam by two magnetic elds to cause it to follow a reverse curve or S shaped path to the target. This bending may conveniently be done by the use of two magnetic elds which are side by side but in opposite directions. The first 56 Cil eld bends the beam away from the initial path and along a different path and the second field adjacent the different path bends the beam back in an opposite direction to the rst bend, so that the final portion of the path of the beam where it strikes the target is parallel to but displaced from the initial portion of its path where it leaves the gun. The absence of keystoning of the pattern on the target is indicative of the parallel relation existing between the initial portion of the path and the final portion of the path of the beam.

I prefer to produce the two magnetic fields by two electromagnets having pole pieces lf3-M, and l5-I6, and exciting coils l1 and I8. These coils surround a portion of the yoke lf3-20, each o1' which connects the respective pole pieces.

The yoke and pole pieces of each electromagnet, preferably of metal having high magnetic permeability, such as silicon steel, are positioned as shown in the drawing and when energized by the coils Il and I8 produce a magnetic field which is substantially uniform between the pole pieces and decreases in intensity radially from` the center line of the pole pieces. That portion of the eld which surrounds the pole pieces constitutes in effect a fringing magnetic field suiiicient to bend the scanning beam through the required angle, usually aboutJ The dimensions of the tube should be so chosen that the distance between the pole pieces of each magnet is less than the distance between the pole pieces of the two magnets in order to minimize distortion of the respective elds. For example, I have obtained goods results with a tube, such as shown in Figure l, having an offset X between the gun axis and target axis of 31/2 inches, a distance of 1% inches between pole pieces, this being the thickness of the tube at the pole pieces, a center to center distance between the fields of the two magnets of approximately 4 inches, and with the center line of the pole pieces IS-M through the norm-al undeected path of the beam. The two magnets were in the proper operating position for this particular tube when so located with respect to the target that the two planes parallel to the target each through the center lines of the pol-e pieces were 0.6 inch apart, that plane through the pole pieces ES-li being 0.4 inch from the surface of the t-arget.

The strength of the magnetic field necessary to bend the beam through the desired angle is dependent upon the radius of curvature of the beam and upon the velocity of the beam. I have found that in a tube of the dimensions above given an electron beam of a velocity corresponding to 6000 volts second anode potential may be given a radius of curvature of 8 cm. with a field strength of approximately 33 gauss, and of 6 cin. with a eld strength of approximately 45 gauss, values easily attainable in practice. With such fields the bending of the beam occurs, as shown in the drawing, in the fringe of the field, which I have found to be desir-able. At such times when the beam s deected from its normal path by the deflection coils Il and i2, the effect of the fringing elds is to maintain the relationship between the two angles of bending such as to maintain the final portion of the path of the beam parallel to the initial portion. Thus, as previously indicated, keystoning of the pattern is substantially eliminated.

A preferred embodiment of the target 2 may be provided by the use of a metal plate 2|, that surface which faces the electron gun being highly polished and having thereon a layer of uorescent material 22, such as willemite, which becomes luminescent under bombardment of the electron beam. The size of the target will depend on other constants of the tube, but I have used targets having an area of approximately 2 square inches. A reiiecting type of target may, of course, be water-cooled if desired, in which case the efficiency of the tube may be somewhat increased. Likewise without departing from the teachings of the invention the uorescent type target such as that shown may be replaced by a target of the thermoelectric type in which radiant energy is liberated by heating the target to incandescence with the electron beam or by any target treated to give .luminescence under the bombardment of an electron beam.

The tube is prepared for operation by energizing the heater of the cathode 5 and applying high positive voltages with respect to the cathode on the first and second anodes while maintaining the grid t at a relatively high negative bias with respect to the cathode. The coils il and i8 vare energized from a source of direct current and the bias on the grid 6 is reduced until there is sufficient current in the beam to generate a spot of light on the fluorescent material 22. The current in each of the coils Il' and I8 is adjusted until the beam strikes the target at the center l anodes is adjusted such as by varying the first I anode potential, to focus the beam into a small spot whereupon the grid bias may be further reduced to give increased light output from the fluorescent screen.

Figure 2 shows a modified form of tube having a target electrode 23 which acts as a light valve and is interposed between a light source 2li and a viewing screen 25. The target electrode 23 may be of the type described by Zworykin U. S. Patent 2,025,143 which includes a plurality of shutters 25 actuated by the electron beam to control the transmission of light from the source .20 to the viewing screen 25. In this modification of my invention the envelope of the tube includes two windows 2l and 28, one on each side of and co-axial with the target electrode 23. The relative position of the target, the electron gun, and the beam bending magnets (not shown) is the same as in the tube shown in Figure l., and the operation is the same, except that the useful light transmitted through the t-arget 23 is controlled by the electron beam. In this modiiication of my invention the electron gun and the beam deiiection means lie wholly outside the path of light from the source 2li to the viewing screen 25, and in addition the target electrode is in such a position that compensation for keystoning is not necessary.

From the foregoing description it will be apparent that various other modifications may be in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim:

l. A cathode ray device having an envelope with an elongated neck section and a window offset from the longitudinal axis of said neck section, an electron gun in said neck section for generating and focusing an electron beam, a target with a luminescent surface opposite said window, means for producing a magnetic eld which is substantially perpendicular to and located near the longitudinal axis of the gun for bending the beam from its initial path as it leaves the gun, means for producing a second magnetic eld which is substantially parallel to said rst eld but displaced therefrom to bend said beam to impinge on said target in a direction substantially parallel to the path of the beam as it leaves said gun, and deflecting means between said gun and said flrst means to deflect the electron beam over the initial portion of its path in two mutually perpendicular directions.

2. A cathode ray device having an evacuated envelope with a Window, an electron gun within said envelope to generate and focus an electron beam, a flat target perpendicular to and laterally displaced from the longitudinal axis of said gun and opposite said window, a magnet having two oppositely disposed pole pieces with the center line of said pole pieces substantially perpendicular to, located near, and on opposite sides of the longitudinal axis of said gun, a second magnet with oppcsitely disposed pole pieces on opposite sides of the plane of said axis and between said target and said window for producing a magnetic field parallel to and opposite in direction to the field of the first magnet to bend said beam to impinge on said target in a direction substantially parallel to the path of the beam as it leaves said gun, the center lines of the pole pieces of said magnets being in a plane between said gun andV said target, and deflecting means between said gun and said first magnet to deflect the electron beam over the initial portion of its path in two mutually perpendicular directions.

3. A cathode ray device comprising an evacuated envelope with a window, an electron gun within and at one end of said envelope for generating a modulated electron beam, a target at the opposite end of said envelope from said electron gun optically exposed to and opposite said window and substantially perpendicular to and laterally displaced from the longitudinal axis of the electron gun and the path of the beam as it leaves said gun, means between said gun and said target for bending the electron beam from the path of the beam as it leaves said electron gun and along a diierent path, means positioned between the window and target, outside said envelope and adjacent said different path to bend the beam in a direction substantially parallel to but laterally displaced from the path of the beam as it leaves said gun to cause the beam to impinge on said target, and means between said gun and the rst of said means for deflecting the electron beam in two mutually perpendicular directions to scan said target.

4. A cathode ray device comprising an evacuated envelope with a window, an electron gun within and at one end of said envelope to generate and focus a modulated electron beam along the longitudinal axis of said electron gun, a target at the opposite end of said envelope from said electron gun having a luminescent surface optically exposed to and opposite said window and facing said gun, said surface being laterally displaced from the longitudinal axis of the electron gun and the path of the beam as it leaves said gun, means outside said envelope for producing between said gun and said target a eld to bend said electron beam from the path as it leaves said gun to impinge on said luminescent surface in a direction substantially parallel to the path of the beam as it leaves said gun, and means between said gun and said bending means for deecting the electron beam in two mutually perpendicular directions to scan said target.

5. A cathode ray device comprising an envelope with an elongated neck section and two oppositely disposed windows offset from the longitudinal axis of said neck section, an electron gun in said neck section for projecting a modulated electron beam, a substantially at target between said windows and facing said gun, the flat surface of said target being substantially perpendicular to and laterally displaced from the longitudinal axis of the electron gun and the path of the beam as it leaves said gun, magnetic means to bend the electron beam from the path as it leaves said gun toward said target and along a different path, magnetic means positioned between said oppositely disposed windows, outside said envelope to bend said beam in a direction substantially parallel and adjacent said different path to the path of the beam as it leaves said gun to cause the beam toimpinge on the surface of said target, and means between said gun and the first of said magnetic means for deflecting the electron beam in two mutually perpendicular directions to scan said target.

ROI-IN TRUELL. 

